P0420 Fix for Subaru WRX / STI

Swapped to a catless or high-flow downpipe and now your WRX throws P0420? That's the rear O2 seeing raw exhaust and telling the ECU the cat is gone (because it is). A tune can disable the code — but if you're not ready to retune, there's a $42 mechanical fix.

The Fix: 90° O2 Sensor Spacer

The Brockett Built 90° O2 sensor spacer threads into the rear (downstream) O2 bung on your downpipe and moves the sensor out of the direct flow, calming the signal enough that the ECU stops flagging catalyst efficiency. CNC-machined, M18x1.5 — the standard Subaru O2 thread.

• Flat-four, single rear sensor — one spacer does it.
• The 90° offset tucks the sensor tight against the downpipe — better clearance than straight extenders in the cramped WRX tunnel.
• Rear sensor only — never the front (wideband) sensor; that one controls fuel trim.
• Works alongside your tune or stock ECU — no flash needed for the code fix.

Fits

WRX (2002–present), STI, Forester XT, Legacy GT — any Subaru with an M18x1.5 rear O2 bung. Also the go-to for GD/GR/VA chassis catless setups.

FAQ

Will this replace a tune?
For the P0420 code, yes. It does not change fueling — your AFRs and boost are untouched.

Fit on an aftermarket downpipe?
Yes — any downpipe with a standard M18x1.5 rear bung.

Legal note: designed for modified/off-road use — check local emissions laws.

Get the 90° O2 Sensor Spacer — $42 →

TIG-welded and machined by Brockett Built in central Illinois. Ships same or next business day. 30-day fitment guarantee.